Load bearing thermal insulation is employed in a number of applications where heat transfer resistance and load bearing capability are required. One such application is evacuated panel enclosures for high temperature batteries. In such applications the insulation must not only have good heat transfer resistance but must also have high compressive strength. A good load bearing insulation compresses very little under load. Unfortunately, conventional materials having the qualities which impart to a material good compressive strength, such as rigidity and hardness, also have poor resistance to heat transfer.
Glass is one material that exhibits both heat transfer resistance and compressive strength. Thermal insulation board has been constructed by stacking layers of glass fiber sheets and adhering them together to form a board of desired thickness. This board can then be cut to fit the configuration of the volume sought to be insulated.
One such known method comprises stacking glass fiber layers held together with a binder or adhesive. Although such a method is relatively simple and inexpensive, the resulting thermal insulation board is inadequate for many types of uses. One reason is that the chemical binder or adhesive between the glass fiber layers provides a conductive pathway for heat to travel through the insulation. Another reason, which arises in a high temperature evacuated panel enclosure, such as a high temperature battery box, is that at such high temperatures, the chemical binder or adhesive breaks down and releases gases which degrade the vacuum resulting in reduced insulation.
Another known method of producing thermal insulation board is disclosed in U.S. Pat. No. 2,745,173--Janos. This method requires compressing a stack of glass fiber layers with a force of at least one atmosphere and preferably greater, and then heating the compressed stack to a temperature above the glass strain temperature but below the glass annealing temperature. Such a process produces good thermal insulation board without the need for chemical binders or adhesives. However insulation board made by this process is inadequate for certain high performance applications. It is believed that the performance of the insulation board produced by this process is compromised because the process causes stresses in the board so that cracks are formed in the compressed boards. This is especially the case in larger boards. Furthermore, this method may cause breakage of some of the glass fibers so that they may become oriented perpendicular to the other fibers and parallel to the heat leak. This reduces the integrity of the insulation board. The relatively large compressive force is believed to increase fiber to fiber contact increasing conductivity and reducing heat transfer resistance of the insulation board.
It is therefore an object of this invention to provide a thermal insulation board which exhibits improved insulation and good compressive strength without use of chemical binders or adhesives.
It is another object of this invention to provide a thermal insulation board which exhibits improved insulation and good compressive strength wherein glass fiber breakage and fiber to fiber contact is minimized.
It is a further object of this invention to provide a method of making thermal insulation board which achieves the objects specified above.